Jung Han. Innovations in Food Packaging

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Modified atmosphere packaging of ready-to-eat foods

193

efficiency. As argon washes into the bottom spaces of a tray in this fashion, it

completely displaces all of the entrained air from the complex structural components

of ready meals, leaving exceedingly low residual oxygen levels in the package. Since

this action of argon depends upon the density of the gas rather than on the pressure

of delivery, low pressure of introduction of gas may be used, limiting the product dis-

ruption by minimizing the flow of gas. Since very low oxygen residuals are obtained

because of the efficient displacement of air by the denser argon, vacuum cycles can be

shortened or eliminated altogether, conferring valuable advantages in line speed of

production.

Modifications of packaging machinery and tools to

accommodate argon

Gas

delivery system

First, since argon is heavier than nitrogen, larger-diameter tubing and pressure lines

must be used to reduce frictional pressure loss and to assure adequate delivery pres-

sures. The use of argon in nitrogen MAP systems will often result in lower line pres-

sures and delivery of an insufficient mass of argon to the point of packaging. Unless

the gas mixer is also recalibrated to accommodate these differences, serious errors

will become manifest in the composition of the final MAP mixture. Furthermore, the

packaging machine solenoid may not be able to deliver sufficient argon to the pack-

age in its normal pulse. The argon systems referred to here use larger-bore supply

lines, mixers, solenoids, and lances, and have redundant pressure-regulation systems

to ensure adequate mass flow rate.

Second, the problems associated with turbulence in nitrogen

MAP

can be avoided

with the application of argon, but the removal of air (oxygen) becomes the key to suc-

cessful application. Adding exhaust vents to

MAP

tooling provides the needed escape

routes for the oxygen. When set up this way, no vacuum cycle is needed for removal

of air, and line speeds are maximized. Upper speeds depend only upon the fill, forming,

and seal functions of the packaging machine.

Films

A wide range of commonly available gas packaging machinery was used in these

example experiments, and all were successfully and easily modified as above to

accommodate argon. A wide range of plastic films and trays were used, and most of

these were found to be satisfactory. The key point was to use highly impermeable films

and trays. A very large array of tests was conducted to determine the best economi-

cal fit of plastic packaging for various product constraints. Generally, excellent results

were obtained with highly impermeable (02 transmission rate 0-5 cm3/day/M2/atm

SPTRH) CPETIPET trays and either high or medium barrier APETPET, EVA1

PET or PPPET film combinations (02 transmission rate 75-100cm3/day/M2/atm

SPTRH). The results reported here were produced on a tray-lidding machine using

heat-sealable peelable topfilms.

194

Innovations in Food Packaging

Example experiments

Sample preparations

Ready meals were produced on site in one of several factories, using a wide range of

different packaging machine types. The types of ready meals included a wide variety

of components, assembled as meat pasta dishes, vegetable pasta dishes, rice-based

dishes with meat and vegetable sauces, curry dishes, meat plus side dishes, vegetarian

entrees, and dishes with added cheese, potatoes, etc. All meals were precooked and

deposited automatically in plastic trays under normal production conditions, and

placed in packaging materials as described above. A medium barrier topweb film was

heat-sealed onto the trays. For the results reported below, all production was carried

out at normal line speed. Gas use measurements confirmed that four to eight times

less gas was used with argon than with nitrogen in each comparative production cycle.

Organoleptic assays

Ready meals were assessed in-house at a dedicated facility by panels of volunteers,

randomly selected, who were asked to score each product using a hedonic five-point

scale (Stone and Sidel, 1993), where 5 was preferred, and 3 was the cut-off for accept-

ability. In order to indicate complete spoilage (score of 0) and thus to exclude toxic

samples from testing, a six-point scale was used in some assessments. Five organolep-

tic parameters were scored

overall acceptability, appearance, flavor, aroma, and tex-

ture. Each of these five parameters was assessed in randomized tests pairing a treated

ready meal and a control. Two tests were used; an independent paired comparison dis-

crimination test, and a paired comparison hedonic acceptance test of the product

against control. A panel was comprised of enough testers to assure that all results were

reproducible, and statistically significant for each assay replicate. Most panels were of

16,24 or 30 people. At least once for each product type, a pooled result of

49 was

obtained by establishing seven panels of seven panelists each; these results reported as

the pooled average of the panels (the differences between the panels were determined

to be non-significant). Statistical significance was set at the 5% probability level (alpha

0.05). Significance was assessed using binomial, chi-squared, or reported proba-

bility tables (Roessler

al.,

1978; Stone and Sidel, 1993). Differences were robust

across the factors of choice of panel and location, date of replication, and method

of test.

addition to the above tests, the products were also assessed organoleptically at

the point-of-pack and post-distribution point by professional factory production per-

sonnel, under the guidance of the

QAIQC

function. Painvise difference tests were

used, and involved the same scales as described above and in sufficient number to

retain statistical significance. Full-scale production was also subject to sample trials

using in-store packages as well as post-production product. Difference tests were con-

ducted on treatment packages and controls sharing similar dating (verified by produc-

tion date codes). Sample packages were obtained off the shelf or taken directly from

Modified atmosphere packaging

ready-to-eat foods

195

distribution. Organoleptic differences displayed robust statistical discrimination as

well as depth.

The samples used in the above tests were prepared, stored, and shipped in accor-

dance with good manufacturing practice and standard protocols for ready meals in

normal retail distribution. Temperatures and hygiene protocols were maintained nor-

mally, and checked throughout the production, storage, and shipment stages. Treatment

samples and controls were subjected to precisely the same specifications, except for

the type of MAP used. All samples were subject to precise gas measurement and

tested for seal integrity. In a separate set of tests, a wide range of handling, microbial,

and temperature-abuse studies were conducted. These tests allowed us to conclude

that argon MAP products do not suffer

greater rate of degradation than those under

nitrogen

MAP,

and nor is the rate of hazard greater.

Finally, as production is ongoing, normal QNQC sampling of all products continues

at an appropriate sampling rate, providing an enormous statistical depth to the results

described.

Microbial assays

Microbial assays were conducted on all products during production and storage

according to good manufacturing practice, as well as in accordance with

and

regulatory standards. Standard methods required by regulatory oversight were used,

and all assays were repeated first in pentuplicate and then in either triplicate or pentu-

plicate. In all cases, total viable count (TVC) was assayed, and in most cases yeasts,

molds,

Pseudomonas

spp.,

Lactobacillus

spp., and

Enterobacter

spp. were counted.

In addition, for selected products, total anaerobes, molds, and pathogens were assayed,

depending upon the specific commercial concerns for each product. Microbial assays

were conducted concomitantly with parallel sensory assessment panels for all products.

All microbial tests were performed by external independent laboratories, and were

always blind-coded.

Results

Relative utility of argon MAP vs Nitrogen MAP in

displacing package oxygen

In order to measure the relative efficacy of nitrogen and argon

removing air from

packages in normal production, ready meals were packaged under optimal production

conditions as described above. In a standard

run,

1000 packages prepared under each

of the two MAP atmospheres (Ar

C02

25;

C02

25) were assayed for

oxygen content after packaging, using an oxygen gas analyzer calibrated to

0.2%

0,.

Figure 12.1 shows the results for a test of residual oxygen in ready meals packaged

under nitrogen

MAP

and argon

MAP.

The results show an average residual

con-

centration of 0.5% for argon MAP, and 98.5% of all packages having less than 1%

196

Innovations in

Food

Packaging

Argon MAP

Nitrogen

MAP

I I

04.5%

0.5-1

1-2% 2-5%

Residual oxygen

Fig-

12.1

Residual oxygen in ready meals packaged under nitrogen MAP and argon MAP.

residual. The results for nitrogen MAP show an average of 5%

residual, with

approximately 50% of packages exceeding the average, and no packages having less

than 2% residual

02.

Similar results were obtained when this experiment was repeated

for each of 130 different ready-meallpackaging combinations. It was in fact possible

with argon to achieve any desired level of residual oxygen concentration by slowing

the packaging cycle on the production line in order to allow time for the argon to flow

through all of the interstitial spaces in the product. Such complete substitution was

unnecessary, however, as it provided removal of only the final 0.143% residual

entrained within the product. It was not ordinarily possible when nitrogen was used to

achieve residual levels of the same order as those obtained with argon, except by the

use of a combination of impractically fast flow rates and excessively retarded packag-

ing cycles.

The variance of the results with nitrogen was very

high.

Under normal line speed pro-

duction, the chance of any given package having an oxygen content more

than

25% dif-

ferent from its neighbor was greater

than

50%. The range of oxygen residuals obtained

was 0-20%, with 95% of the packages having greater

than

2%.

addition, the entire

distribution was skewed strongly toward higher percentages. Under practical conditions,

it was not possible to enforce a specification of 5% residual oxygen. Under argon MAP,

on the other hand, the range of oxygen residuals obtained was far smaller, &2%, and

showed no skewness. The mean target of 0.5%

was obtained in 98.5% of all pack-

ages.

specification of

.O% residual oxygen was set and met for all products.

Effectiveness of argon

MAP

in controlling

microbial growth in ready meals

The effect of argon MAP compared to nitrogen MAP or air control on microbial growth

in ready meals was assessed in over 130 types of ready meals by random sampling of

ready meals produced under normal operating conditions. Gas composition was

checked on all samples.

Modified atmosphere packaging of ready-to-eat foods

197

Figures 12.2a-c show representative results for the effect of argon MAP upon

microbes across three different ready-meal types. Figure 12.2a shows the total viable

count (TVC) in a pasta carbonara ready-meal product under air, nitrogen MAP

(Nz

C02

25) or argon MAP

(Ar

C02

25) atmosphere. While nitrogen

MAP is effective in inhibiting microbial growth, significant additional repression of

microbial growth is seen with argon MAP. As the shelf-life cut-off for this product is a

TVC of lo7, the shelf-life extension using nitrogen

MAP

was found to be 1 day (12%),

while with argon MAP it is 6 days (120%) longer than air control. In another example

(Figure 12.2b), a vegetable pasta ready meal, the shelf life of argon MAP was 15 days

(88% increase), while an 8-day shelf life was obtained under nitrogen

MAP.

It was not

possible to obtain shelf-life extension for a curried vegetable and rice-based ready meal

using nitrogen MAP, as the air entrained within the rice base under the sauce could not

be effectively flushed out. The problem was solved using argon, to provide a 13-day

shelf life compared to 4 days for the air control (Figure 12.2~).

The effect of argon MAP on TVC is replicated with

Pseudomonas

spp.,

Enteromonas

spp., lactic acid bacteria, yeasts, and molds.

a typical product, for

Pseudomonas

spp., argon held CFU below

lo3

throughout an 18-day shelf life, whereas the air control

passed that mark on day 4 and the nitrogen

MAP

passed it on day 6. For

Enteromonas

spp., both air and nitrogen passed 1

on day 10. For yeasts, argon

MAP

provided sup-

pression below the lo5 cut-off throughout shelf life for all products, whereas the air

control failed at 50% of life and nitrogen MAP failed at 67% of life for more

than

three-quarters of the products. Lactic acid bacteria were very well controlled by argon

MAP in about half of all products overall, especially those with acidic sauces. Further

assessment is required, although lactic acid bacteria were usually not a determinant of

shelf life in the ready-meal products.

Ability of argon MAP to preserve organoleptic characteristics

of ready meals

Organoleptic assessment of argon MAP as compared to nitrogen MAP and air control

was carried out for all products. Tests were initially carried out at each sampling day

post-packaging, but later tests focused on the peak and end of the shelf life. As an

example, Figure 12.3a shows the results for the pasta carbonara ready meal at peak of

life, where panelists preferred the argon MAP product in direct choice tests for all

parameters. Differences were significant and reproducible across multiple panels.

Differences found between nitrogen

MAP

and air control samples were more difficult

for panels to discriminate, and discrimination among samples along individual

parameters were more variable. For both a vegetable pasta ready meal and a meat

ready meal presented at day 6, results (Figure 12.3b and 12.3~) show a significant

preference for the argon MAP over the nitrogen MAP product. Figure 12.4 shows the

progression of preference discrimination through storage for a complex ready meal

with a 14-day (in argon MAP) shelf life.

this example, scores from three panels

are averaged, and a score of 10 is the limit for saleability. The positive effect of

argon upon all organoleptic parameters increased steadily throughout the shelf life of

the product.

198

Innovations in Food Packaging

1.00E+02

1.00E+01

12345678910

(a) Day after packaging

1.00E+01

0 12 3 4

6 7 8 9 10111213141516

(b)

Day after packaging

Air

-m-

Argon MAP

0 12 3 4 5 6 7 8

10111213141516

Day after packaging

Figum

12.2

(a) Microbial growth in a pasta carbonara ready meal under different MAP atmospheres;

(b) Microbial growth in a vegetable ready meal packaged in nitrogen MAP and argon MAP; (c) Microbial

growth in a curried vegetable and rice ready meal packaged in argon MAP and air control.

Modified atmosphere packaging of ready-to-eat foods

199

Aroma

Air

Nitrogen MAP

Flavor

Appearance

Sensory score

Texture

Aroma

Flavor

Appearance

Overall

acceptabilip

3.5

Sensory score

Texture

Aroma

Flavor

Appearance

Overall

acceptability

Sensory score

Figure

123

(a) Organoleptic scores for a pasta carboGara ready meal packaged under different

modified atmospheres; (b) Sensory scores for a vegetable ready meal packaged in nitrogen

MAP

and argon

MAP;

MAP

and argon

MAP.

200

Innovations in Food Packaging

Day after packaging

Figum

12.4

Changes in ready meal organoleptic score over time.

Air

Nitrogen MAP

7 8

16 17 18 19 20

Ready meal samples

Shelf life -Air

Oraanole~tic shelf life

Araon MAP

Microbial shelf life -Argon MAP

Fipm

12.5

Shelf-life improvement of

ready meals in practice using argon

MAP.

Practical application of argon MAP in extending the

shelf life of ready meals

Figure 12.5 shows the average shelf-life improvement of ready meals in practice when

using argon MAP. In the initial assessment of an argon installation at one factory,

20 disparate ready meals were assessed for both microbial and organoleptic limitations

on shelf life. Current practice was packing in air, and nitrogen

MAP

had been found

not to significantly add to shelf life, as results were excessively variable. Installation

of an argon system provided an immediate increase in shelf life as measured either by

organoleptic acceptability or by microbial cut-off (as indicated in Figure 12.5). All

shelf-life specifications were increased for the 20 ready-meal products. The average

specified shelf life of the packaged products before the introduction of argon

MAP

was 9 days. With argon

MAP,

the microbial shelf life was increased by an average of

an extra

days

(56%)

average of 14 days, while the

organoleptically-determined

shelf life was increased by an average of

extra 4 days (44%) to 13 days. These

results were extended to a full range of over 100 products at several manufacturing

Modified atmosphere packaging of ready-to-eat foods

201

sites. The resulting extension of the shelf life provided retailers with an extra level of pro-

tection against food safety concerns, and allowed for a complete reorganization of pro-

duction scheduling and distribution logistics. Besides tlvs significant improvement in the

cost profile of the production facilities concerned, it was possible to increase profitabil-

ity significantly through the control of wastage and out-of-stock at the retail store level.

Conclusions

Oxygen catalyzes the oxidative degradation of foods, particularly of flavor, aroma,

and color components. Microbial growth in packaged foods, especially under aerobic

atmospheres, causes spoilage of the product. Modified atmosphere packaging (MAP)

is effective in lowering the residual oxygen levels in ready-meal packages. Nitrogen,

usually combined with COz, has been used for many years as the MAP gas of choice

in controlling oxidation of foods, and extending its organoleptically determined shelf life.

Carbon dioxide confers antimicrobial action to the MAP atmosphere, and extends the

microbially determined shelf life of foods by inhibiting the growth of micro-organisms.

Owing to the different physical properties of argon, especially its higher density,

it is far superior to nitrogen in removing andlor excluding oxygen from packages,

and much lower levels of residual oxygen may be obtained in practical application.

Since argon, unlike nitrogen, inhibits oxidase activity, argon-based MAP mixtures

effectively depress respiration in microbes. Less C02 is therefore needed in argon

MAP

than

nitrogen MAP to control microbial growth, and using argon combined

with C02 provides a higher level of antimicrobial action than that obtainable with

nitrogen MAP. As C02 is destructive to food products, the use of less C02 means that

there will be a better preservation of food quality. Thus, while nitrogen

MAP

is a cost-

effective means of preserving foods, argon

MAP

is significantly better at controlling

oxidation and microbial growth, and results in a significant improvement in the shelf

life of a wide range of ready meals. Argon MAP has also been successfully tested,

developed, and profitably commercialized for the preservation of ready meals and

other products.

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